Impact of crushing shape and geopolymerization on reclaimed concrete aggregate for recycling in the flexible pavement: an enhanced circular economy solution

This study evaluates the quality and mechanical performance of various shapes of reclaimed concrete aggregates (RCAs), including elongated, cone-shaped, angular, and ordinary mix, in comparison to fresh aggregates (FAs). The RCAs are treated with fly ash-based geopolymer, and the effects of various percentages of geopolymer content on the geotechnical properties of the modified RCAs are investigated. The study finds that the angular shape of an RCA exhibits more strength than other shapes, and the use of fly ash-based geopolymer significantly improves the mechanical characteristics of the modified RCAs by up to 10%. The research also shows that the use of fly ash-based geopolymer-treated RCAs in the subbase layer of flexible pavements can lead to a reduction in the required thickness of the base layer, resulting in significant cost savings and curtailment of CO2 emission compared to the use of FAs. Abbreviations: AASTHO: American association of state highway and transportation officials; A: Area of cross-section of flexible pavement; ACV: Aggregate crushing value; AIV: Aggregate impact value; BS: British standard; ASTM: American society for testing and materials; C_c : Co-efficient of curvature; CO₂: Carbon dioxide; C_u : Co-efficient of uniformity; D₅₀ : Mean grain size; DCW: Demolished concrete waste; CBR_(s): Soaked California bearing ratio; FA: Fresh aggregate; FI: Flakiness index; EI: Elongation index; GC: Geopolymer content; GSD: Grain size distribution; k: Co-efficient of permeability; LAAV: Los Angeles abrasion value; m: Drainage co-efficient; m₂ : Drainage co-efficient of base layer; m₃ : Drainage co-efficient of subbase layer; m_a : Dry mass of additive; M_R : Resilient modulus; M_R(B) : Resilient modulus of base layer; M_R(SB) : Resilient modulus of subbase layer; M_R(SG) : Resilient modulus of subgrade layer; m_RSM : Dry mass of reclaimed subbase material; NAOH: Sodium hydroxide; Na₂SiO₃: Sodium silicate; PKR: Pakistani Rupees; R: Reliability; RCA: Recycled concrete aggregate; S_o : Overall standard deviation; SN: Structural number; USCS: Unified soil classification system; NCHRP: National cooperative highway research programme; NHA: National highway authority; W₁₈ : 18-kip equivalent single axle loads; US$: US Dollars; ξ_OGC : Optimum geopolymer content; w_opt : Optimum moisture content; γ_dmax : Maximum dry unit weight; a: Structural layer co-efficient; a₁ : Structural layer co-efficient of asphalt concrete; a₂ : Structural layer co-efficient of base layer; a₃ : Structural layer co-efficient of subbase layer; ΔPSI: Present serviceability index; t_b : Thickness of base; t_sb : Thickness of subbase; δ: Total material weight.